The present invention relates generally to driving apparatus comprising axles and, more particularly, relates to an integrated hydrostatic transaxle comprising a differential lock mechanism.
As is known, the use of a standard differential assembly in the transmission system of a driving apparatus, such as a motor vehicle, allows the wheels to spin at different speeds. The differential assembly, or differential gear unit, differentially connects a pair of axle shafts. In the case of a vehicle traveling in a straight line, the axle shafts connected to the standard differential assembly will rotate at the same speed. When a turn or curve is encountered, the axle shaft nearest the inside of the turn will slow in rotational speed while the outer axle shaft will simultaneously increase in rotational speed. As such, the wheels, driven by the axle shafts, are prevented from scuffing the surface across which they travel.
While the standard differential assembly serves an important function in the operation of a transmission system, the standard differential of simple construction has difficulty operating under certain conditions. For example, when a first one of the drive tires is disposed in wet, muddy, or loose soil conditions, or when the first drive tire has been partially or completely removed from contact with the ground, the coefficient of friction under the first drive tire will be substantially lower than that associated with the second drive tire. This resulting frictional imbalance will tend to cause the second drive tire to remain stationary while the first drive tire will spin without moving the vehicle.
To solve the problem of loss of traction in transaxle systems, a variety of techniques have been developed to reduce the tendency of the transaxle to differential. For example, U.S. Pat. No. 5,897,452 to Schreier, et. al., issued Apr. 27, 1999, discloses a means for preventing free spinning of one of the driven shafts of a transaxle without interfering with the normal differential capability of the transaxle. In particular, the ""452 patent discloses the use of spring washers to inwardly force the planet bevel gears of the differential into contact with frictional surfaces that reduce the tendency of the bevel gears to move and thus resists relative rotation of the axle shafts. In this manner, the frictional force limits the free spinning of one axle shaft relative to the other thereby improving traction.
While the friction inducing means disclosed in the ""452 patent works for its intended purpose to provide transaxles with a controlled traction differential assembly, this assembly can still allow relative motion of one axle shaft with respect to the other under certain higher output torque operating conditions. For these conditions, a method of locking the two axle shafts together is required. Traditionally such locking methods have not generally been available to smaller transaxles due to space constraints. The present invention improves on the prior art.
An object of the present invention is to provide a differential system in an axle driving apparatus. In particular, it is an object of the present invention to provide an integrated hydrostatic transaxle having a differential locking mechanism assembly which will provide the hydrostatic transaxle with the benefits and advantages which have accrued to other types of transaxles that use differential locking mechanism assemblies.
Another object of the present invention is to provide a compact lockable differential mechanism supported by a drive gear rather than a separate housing. A further object of the present invention is to reduce part wear by reducing movable parts and integrating part of the locking system with the differential housing.
An object of the present invention is to provide a locking fork supported in a housing containing the differential assembly. A further object of the present invention is to provide this support by positioning the fork shaft at one end in one blind hole and at another end in one bushing.
Another object of the present invention is to simplify installation of a lock mechanism for a differential, whereby the transaxle may be retrofitted with the locking mechanism. A further object is to provide this simplified installation by providing sufficient clearance among components by positioning the shift mechanism and the lock mechanism away from the differential components. A further object is to reduce time and costs by reducing the number of installed parts and by simplifying the installation thereof. This object is particularly achieved by utilizing differential pins as part of the locking mechanism. A further object is to provide a more compact locking differential system through use of the differential pins.
A further object is to reduce wear and contact stresses on a sleeve of the locking mechanism. Another object is to reduce down-time and repair costs associated with fatigued parts by providing rotatable guides for connecting the sleeve with a fork; the fork being used to adjust the position of the sleeve.
Another object of the present invention is to provide a system less susceptible to inadvertent activation or damage to a control arm. A further object of the present invention is to reduce this susceptibility to damage by positioning the control mechanism in predetermined safe positions, preferably positions other than on the bottom side of a housing. Thus, even when good clearance is not obtainable, one avoids impacting curbs, debris, ground cover, etc. with the control arm when the arm is safely positioned in other locations.
In accordance with these and other objects, an integrated hydrostatic transaxle is provided. Generally, the transaxle comprises a hydrostatic transmission including a center section on which is supported a hydraulic pump unit and a hydraulic motor unit and a motor shaft drivingly connected to the hydraulic motor unit. A differential assembly is drivingly linked to the motor shaft for use in driving a pair of axle shafts. The differential assembly will be described in the context of spur gears.
The differential assembly comprises a pair of spur gears contained within a housing assembly formed by a bull gear and a separate plate. Drivingly associated with the spur gears of the differential are two gears matingly attached to two separate axle shafts, where one of these two gears has formed with it a spur gear (or sleeve gear) that extends beyond the housing formed by the bull gear. Matingly associated with this separate spur gear is a sleeve which is moveable to engage pins attached to the differential assembly. Such engagement thus prevents movement of the differential assembly with respect to the axle shaft, thereby locking to the two axle shafts together.
In a preferred embodiment of the invention, the differential housing is composed partially of the bull gear. Other embodiments may use separate housing parts affixed to the bull gear. In a preferred embodiment one of the spur gears is positioned in the differential assembly and an extension, e.g., a sleeve extending from the spur gear or from the axle, is positioned, at least partially, external to the differential assembly. Other embodiments may use two separate gears associated by splines, for example, on the shaft, or matingly associated with each other in some other fashion to accomplish the same purpose.
A better understanding of these and other objects, advantages, features, properties and relationships of the invention will be obtained from the following detailed description and accompanying drawings which set forth an illustrative embodiment and is indicative of the various ways in which the principles of the invention may be employed.